regulator: pfuze100: Use devm_register_sys_off_handler()

Use devm_register_sys_off_handler() that replaces global
pm_power_off_prepare variable and allows to register multiple
power-off handlers.

Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>